Wireless handsets are increasingly required to operate in multiple modes (e.g. Global System for Mobile Communications (GSM) general packet radio service GPRS, enhanced data rates for GSM Evolution (EDGE), code division multiple access (CDMA), wideband CDMA (WCDMA) high-speed uplink packet access (HSUPA), long term evolution (LTE). Wireless handsets are also increasingly required to operate in multiple frequency bands (e.g. 700, 800, 900, 1700, 1800, 1900, 2100, and 2600 MHz bands).
Multi-mode and multi-band wireless handsets typically contain a separate dedicated power amplifier (PA) for each individual mode and band of operation. For example, a quad-band GSM and tri-band WCDMA handset will often have one GSM power amplifier covering digital cellular service (DCS) and personal communications service (PCS) bands, one GSM power amplifier covering United States cellular and European cellular bands, one WCDMA power amplifier for United States cellular band, one WCDMA power amplifier for PCS band, and one WCDMA power amplifier for International Mobile Telecommunications-2000 (IMT-2000) band. Often the two GSM band power amplifiers will be packaged into a single power amplifier module, yet these modules typically contain two separate power amplifier circuits, one for the higher bands and another for the lower bands. Having five complete discrete power amplifier circuits as in this example has a significant size and cost impact on the overall wireless handset design.
Wireless handsets use dedicated power amplifier circuits for each band and mode to optimize input and output matches to achieve the best linearity and/or efficiency for the given mode or band of operation. Although the main transistor of the amplifier circuit is broadband, the bandwidth of the amplifier circuit is typically made narrower by the input and output matching circuits. Therefore, to achieve acceptable linearity and efficiency, often power amplifier circuits using fixed matching networks tuned for the different bands and modes of operation are used in a wireless terminal. Using fixed matching networks, a semiconductor power transistor device can only efficiently transmit RF signals in a single mode and a single band.
While some power amplifiers may be operated in two modes which have similar bands, compromises are often made for one mode or both modes due to different modulation schemes. Further, the GSM and CDMA modes present challenges because the GSM system the power amplifier transistor operates in saturated region, whereas in CDMA the transistor has to operate at a more linear region in continuous time. The different operations lead to very different impedance matching solutions at the output of a PA. In a fixed impedance system design, such as commonly used 50 ohm system, a fixed matching network cannot satisfy both modes simultaneously.
In terms of frequency coverage, a single power amplifier circuit typically can only cover either a low band (e.g. 800-900 MHz), or a high band (e.g. 1700-1900 MHz), or a Universal Mobile Telecommunications System (UMTS) band (2100 MHz). Again, the load impedance presented at the output of the power amplifier transistor can be quite different at various operating frequencies. Thus, a single fixed matching network cannot usually provide optimum matching for all potential frequency bands simultaneously. Therefore, multiple PAs are often required in a multi-mode multi-band wireless handset.
The ever decreasing form factor and ever increasing functionality demanded of wireless terminals creates conflicting challenges on front-end devices like number of PAs that can be installed. Currently, board space constrains limits the number of PAs on even the most complicated handset units. Despite much effort within the industry to aggressively reduce device sizes, the fundamental physics and fabrication challenges prohibit the device size shrink to keep pace with the number of added functions within the wireless handset. Not only do additional PA devices cost more board space, the peripheral passive components around each PA device also occupy board space and increase proportionately with the number of PAs used.
Many solutions have been used to improve PA performance and efficiency therefore battery life. For example, PA modules may implement as Envelope Modulation/Envelope Tracking, bias modulation, and Digital Pre-distortion. Also, the wireless networks utilize closed-loop power management schemes to adjust wireless handset PA output levels in order to reduce potential interference and save wireless handset power. All these solutions change the output characteristics of a PA. However, fixed matching networks may prevent full utilization of these solutions.
Therefore, there is a need in the art for an improved power amplifier module. In particular, there is a need for a power amplifier module that is capable of amplifying multiple modes and multiple bands at multiple power output levels.